[Neurobiological mechanisms of muscle pain referral.]

In contrast to pain from the skin, muscle pain is often referred to regions remote from the lesion. For instance, trigger points in neck muscles can elicit pain in the head. The convergence-projection theory of Ruch is still the central concept for the explanation of pain referral. The basis of the theory is that a dorsal horn neuron has convergent input from two different body regions. Because of the convergence, thalamic neurons cannot localize the origin of the dorsal horn activation. Basically, the referral of pain is a mislocalization of pain. Some aspects of muscle pain referral in patients cannot be explained by the convergence-projection theory. Therefore, the present paper presents another mechanism, which consists in acute changes in dorsal horn synaptic connections following nociceptive input from muscle. Results from animal experiments indicate that dorsal horn neurons possess ineffective synaptic connections with the body periphery, which become effective under the influence of a painful stimulus and lead to a mislocalization of pain. The neuropeptide substance P is probably involved in the changes in functional organization that occur in the dorsal horn during muscle pain and its referral.